well, if only playing movies/tv shows, the 4gb is fine, and either windows 7 or 8 is fine aswell, its more of a personel preferance thing. if you can afford 8gb ram go for it, but you should be ok with 4gb.
I have a separate BluRay player for all my DISC needs, and use the Western Digital drive for everything else.
- can copy/paste files via network (or move USB drive to PC then back)
- supports ISO image files for DVD or BluRay (ripped some DVD's for my sister's kids using DVD Decrypter, then Imgburn.)
- plays all the MKV, MP4 etc files I've found
- Netflix etc.
- adds Thumbnails, and description if desired for movies
*You want something that can easily turn on and off, and navigate with a remote control.